The present invention relates to the field of solid state lighting, and in particular to an arrangement driving one or more LED strings wherein a short circuit to ground is detected and driving is interrupted.
Light emitting diodes (LEDs), and in particular high intensity and medium intensity LED strings, are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications.
In a large LCD matrix display, and in large solid state lighting applications, such as street lighting and signage, the LEDs are typically supplied in a plurality of strings of serially connected LEDs, the strings connected to a power source in parallel at least in part so that in the event of failure of one string at least some light is still output. The constituent LEDs of each LED string thus share a common current.
Similarly LED based lighting, also known as solid state lighting, is favored due to its high efficiency, long life, mechanical compactness and robustness, and low voltage operation. Because of the limited power capacity of a single LED device, in most applications multiple LED's are connected in series to form an LED string, wherein all of the constituent LEDs share a common current. Furthermore, as described above, multiple LED strings are typically provided either in parallel, or separately driven, to produce the desired overall light intensity.
LED strings exhibit a particular voltage to current relationship, wherein for a voltage below a minimum operating voltage no appreciable current flows, and for voltages exceeding the minimum operating voltage the current follows an exponential curve responsive to the voltage. As a result, driving of LED strings is typically performed by a constant current source, i.e. a driver which produces a fixed output current irrespective of the voltage developed across the load. Such a constant current source can be of a linear type or of a switching type, wherein the LED current is maintained at a constant pre-determined level with a control mechanism.
In practical mass production of such a multiple LED string lighting apparatus, and especially in display backlight applications, there is a possibility that at some point a LED string is shorted to a metal chassis. Such a short circuit would cause an electronic device failure or a safety hazard when electrical power is applied to the system. The constant current source will continue to drive the designed current through the normal LED strings, with the current of the shorted string bypassing the sense element, and thus not being part of the control feedback loop. Such a situation is dangerous but cannot be detected by the normal sense feedback, which is arranged to ensure that current through the sense element meets a pre-determined target. Thus, the loop of the prior art leads to a hazardous situation in the event of such a short circuit.